As is well known in the gas turbine engine technology, the advancement of high performance aircraft has imposed heavier loads on the turbine and compressor rotors. Thus, it is incumbent on the design of the component parts of these rotors that the stresses do not impair the structural integrity of the rotor and the rotor components. One of the problems that have been evidenced in the firtree attachments for blades is that the attachments have flat surfaces on the bearing surfaces at the interface of the blade and its broach slot in the disk of the rotor. Since the surface is flat the angle at the bearing surface is the same nominal angle through the bearing surface when the interface attempts to make full contact at all levels of loadings on the rotor. The consequence of imposing a heavy load on this flat surface is that this produces high bearing and shear stresses at the edges of the bearing surface. These high edge loadings increase the concentrated stress in the fillets that are either on the blade or on the disk adjacent to the bearing surface which can adversely affect the longevity of these components.
In addition to these problems noted in the immediately above paragraph, because each bearing surface has its own tolerance, stresses at one edge of the bearing area may be much higher or lower than at the other edge which causes more scatter and again is life limiting. To compensate for these abnormalities, the designer of the attachment must design the hardware so that the allowable stress is reduced in order to achieve the maximum design life. Obviously, in order to attain the allowable stress and reduce the design allowable stresses, the attachment weight is increased. As one skilled in this art appreciates, the weight of the components of the engine adversely affect the weight to thrust ratio with a consequential deficit in engine performance.
We have found that we can obviate this problem and enhance attachment life of the blade firtree and/or disk broach slot. In accordance with this invention selective bearing surfaces at critical locations is judiciously designed with a discrete convex or "crowned" bearing surface.